1. Field of the Invention
The present invention relates to a bipolar transistor and a power amplifier. More particularly, the present invention relates to a bipolar transistor which is formed as an active element on a semiconductor integrated circuit for power amplification which is used in a signal transmitting section of a mobile radio terminal (a mobile telephone, etc.) which uses a radio frequency band, and a power amplifier employing the bipolar transistor.
2. Description of the Background Art
In recent years, a semiconductor integrated circuit for power amplification which employs, as an active element, a bipolar transistor made of a compound semiconductor which enables a radio frequency operation and a positive power supply operation, is widely used in a signal transmitting section of a mobile radio terminal, such as a mobile telephone or the like. Particularly, a heterojunction bipolar transistor (hereinafter abbreviated as HBT) is widely used in which a base layer thereof is a p type GaAs layer, and an emitter layer thereof in a heterojunction with the base layer is an AlGaAs or InGaP layer having a large band gap. The reason why the HBT is adopted among bipolar transistors is that holes of the base layer can be prevented from flowing backward into the emitter layer and recombining with electron carriers of the emitter, thereby increasing the efficiency of injection of electron carriers from the emitter to the base, resulting in a high-efficiency operation of the transistor.
The HBT generally has a multi-finger structure in which one or a plurality of stripline-type emitter fingers are arranged side by side. See, for example, Japanese Patent Laid-Open Publication No. 6-342803 and Japanese Patent Laid-Open Publication No. 2002-110904. FIGS. 10A and 10B are a plan view and a cross-sectional view illustrating an exemplary HBT structure in which one emitter finger is provided. In the structure of the conventional HBT of FIGS. 10A and 10B, one emitter finger (an emitter layer 104 and an emitter electrode 101) and two base fingers (base electrodes 102) provided on both sides of the emitter finger, are interposed between two collector fingers (collector electrodes 103). Typically, an output power of the HBT is determined, depending on an area of the emitter. Therefore, in the case of the HBT having one emitter finger, a length L of the emitter finger needs to be large so as to achieve a high output power. However, the emitter finger needlessly occupies a semiconductor chip area.
To avoid this, a plurality of emitter fingers are provided so as to achieve a high output power of the HBT without increasing the emitter finger length L. FIGS. 11A and 11B are a plan view and a cross-sectional view illustrating an exemplary HBT structure in which four emitter fingers are provided. In the structure of the conventional HBT of FIGS. 11A and 11B, four emitter fingers and five base fingers provided on both sides of the emitter fingers, are interposed between two collector fingers. In the case of the HBT of FIG. 11A, since the emitter area can be occupied by the plurality of emitter fingers, a high-output power HBT can be achieved using the emitter fingers each having a length L smaller than that of the HBT of FIG. 10A.
A multi-cell structure in which the multi-finger structure HBT having excellent radio frequency characteristics is considered as a unit cell and a plurality of the HBT cells are connected in parallel to combine outputs thereof, is widely used as a power-amplification HBT structure. Exemplary power-amplification HBTs having the multi-cell structure are illustrated in FIGS. 12A and 12B. FIG. 12A illustrates an exemplary multi-cell structure in which a plurality of HBT cells each having one emitter finger are provided. FIG. 12B illustrates a multi-cell structure in which a plurality of HBT cells each having four emitter fingers are provided. In FIGS. 12A and 12B, the collector outputs of the HBT cells are collectively connected to a common collector conductor 100, and the emitters of the HBT cells are collectively connected to a common emitter conductor 110. The emitter conductor 110 is provided with a via hole 120 and is grounded via the via hole 120.
However, in the case of such a multi-cell structure, the following points should be noted. In the power-amplification HBT, each HBT cell generates heat due to high current density. However, the HBT cells do not uniformly generate heat, so that a failure occurs in the operation of the HBT cell due to non-uniformity of heat generation between each HBT cell. More specifically, the HBT cell having a temperature higher than the surrounding having a high increase in temperature further generates heat due to positive feedback (thermal runaway), finally leading to breakdown. Also, when heat generation is not uniform between each HBT cell, none of the HBT emitters effectively functions, resulting in a deterioration in radio frequency characteristics.
Therefore, in order to correct the non-uniformity of heat generation between each HBT cell, a method of inserting an external base resistance 130 between a DC bias supply line 150 for supplying a DC bias to each HBT cell and a base conductor 140 of each HBT cell, is employed as clearly illustrated in FIGS. 12A and 12B. By insertion of the external base resistance 130, an increase in base current of the HBT can be suppressed, thereby making it possible to avoid thermal runaway even if temperature increases. The external base resistance 130 is used to stabilize an operation of the HBT cell, and is generally called a base ballast resistance. Note that, by providing a ballast resistance to the emitter of each HBT cell, an increase in collector current is also suppressed, thereby making it possible to avoid thermal runaway. However, recently, a base ballast resistance is used more often than a collector ballast resistance is, in view of the narrowness of a value range within which the ballast resistance can take.
As described above, the conventional power-amplification HBT employing a base ballast resistance takes measures against the non-uniformity of heat generation between each HBT cell, however, the non-uniformity of heat generation in each HBT cell is not taken into consideration. The non-uniformity of heat generation in each HBT cell refers to the non-uniformity of heat generation between each emitter finger, which occurs due to the following cause.
In the HBT cell having four emitter fingers of FIG. 11A, when all of the emitter fingers generate almost the same amount of heat, two central emitter fingers are affected by heat from two outer emitter fingers, so that the two central emitter fingers have a higher temperature. Specifically, heat generation distribution regions of the two central emitter fingers overlap heat generation distribution regions of the two outer emitter fingers, so that the two central emitter fingers have a higher temperature (see FIG. 13). Note that, in order to correct the problem, it is considered to provide a sufficient interval between each emitter finger to completely separate the heat generation distribution regions from each other. In this case, however, the area of the HBT cell is increased, resulting in newly arising problems: an increase in chip area; a deterioration in radio frequency characteristics; and the like. Therefore, it is not practical.
In the case of the HBT cell of FIG. 11A, since the five base fingers are provided on both the sides of the four emitter fingers, non-uniformity is likely to occur between each finger in terms of a contact resistance of the base electrode and the base layer of each base finger. Therefore, non-uniformity occurs in the injection amount of a base current, resulting in non-uniformity between each emitter finger.
This problem similarly exists in the HBT cell having one emitter finger and two base fingers of FIG. 10A. Specifically, non-uniformity is likely to occur between the two base fingers in terms of a contact resistance of the base electrode and the base layer of the two base fingers. Particularly, in this HBT cell, the base finger is long, corresponding to the emitter finger, so that the non-uniformity of the contact resistance increases in the length direction. Due to non-uniformity of the base resistances of both the sides, non-uniformity in the operation of the HBT cell increases. For example, a current flowing through the emitter finger is larger in a portion closer to the right base finger than in a portion closer to the left base finger.
Further, in the HBT cell, a base-collector parasitic capacitance is high, and power feedback due to the parasitic capacitance causes a gain deterioration in radio frequency band applications. The radio frequency characteristics of the HBT are determined, mainly depending on a parasitic capacitance occurring between the base layer and the collector layer, and the parasitic capacitance is proportional to a base mesa width W1 illustrated in FIGS. 10B and 11B. This is because the base-collector capacitance is proportional to areas between the base layers sandwiching the collector layer, and the collector layer. In order to reduce the base mesa width W1, it is necessary to reduce areas other than a required emitter area to the extent possible. However, the HBT cell structure of FIG. 11B has five bases for four emitters and the HBT cell structure of FIG. 10B has two bases for one emitter, i.e., the base area is invariably larger by an area corresponding to one base electrode than the required emitter area. Thus, in the conventional HBT cell structure, there is a limitation on the reduction of the base mesa width W1 due to the influence of the base finger, resulting in a deterioration in gain characteristics in a radio frequency band.
On the other hand, in the power-amplification HBT having the multi-cell structure, the value of the external base resistance 130 is large as illustrated in FIGS. 12A and 12B. Therefore, when the power-amplification HBT is actually integrated on a semiconductor, a large area needs to be secured for an external base resistance, resulting in a large chip area, i.e., an increase in cost.